An aluminum alloy magnetic disc used in a storage device for computer is produced from an aluminum alloy substrate of JIS 5086 (3.5 to 4.5 mass % Mg, 0.50 mass % or less Fe, 0.40 mass % or less Si, 0.20 to 0.70 mass % Mn, 0.05 to 0.25 mass % Cr, 0.10 mass % or less Cu, 0.15 mass % or less Ti, and 0.25 mass % or less Zn, the balance being obtained from Al and unavoidable impurities) having excellent plating properties and excellent mechanical properties as well as excellent processability. Further, the aluminum alloy magnetic disc is produced from an aluminum alloy substrate having the contents of Fe, Si, and the like, which are impurities in the JIS 5086, restricted to render an intermetallic compound in the matrix small for the purpose of removing a pit defect caused due to the removal of an intermetallic compound in the pre-plating treatment step, or an aluminum alloy substrate having intentionally added thereto Cu or Zn in the JIS 5086 for the purpose of improving the plating properties, or the like.
A general aluminum alloy magnetic disc is produced as follows. First, an aluminum alloy sheet is prepared and then an annular aluminum alloy substrate (disc blank) is formed therefrom, and subjected to lathing and grinding, and then annealed to obtain an aluminum alloy substrate. Then, the obtained aluminum alloy substrate is subjected to plating, and further a magnetic material is deposited on the surface of the aluminum alloy substrate to produce an aluminum alloy magnetic disc.
For example, an aluminum alloy magnetic disc using the above-mentioned JIS 5086 alloy is produced through the production steps described below. First, an aluminum alloy having desired chemical components is cast, and the resultant ingot is subjected to hot rolling and then subjected to cold rolling to form a rolled material having a thickness required for a magnetic disc. The rolled material is, if necessary, annealed during the cold rolling or the like. Then, an annular piece is punched from the rolled material, and, for removing a strain or the like caused in the above production step, the annular aluminum alloy sheets are stacked and subjected to pressure annealing in which the stacked sheets are annealed while pressurizing the both sides to flatten the sheets, preparing a disc blank.
The thus prepared disc blank is subjected to lathing and grinding as a pretreatment and then, for removing a strain or the like caused in the processing step, the disc blank is heated to form an aluminum alloy substrate. Then, the aluminum alloy substrate is subjected to degreasing, etching, and zincate treatment (Zn substitution treatment) as a pre-plating treatment, and further subjected to Ni—P electroless plating, which is a hard non-magnetic metal, as an undercoat treatment. Finally, the Ni—P electroless plated surface is polished, followed by sputtering of a magnetic material, to produce an aluminum alloy magnetic disc.
By the way, in recent years, as there are needs of multimedia and the like, magnetic discs are required to have an increased capacity and an increased density. For further improving the recording density of the magnetic disc, it is necessary to further reduce the flying height of a magnetic head from the magnetic disc and further stabilize the distance between them. For achieving this, the aluminum alloy substrate for magnetic discs is required to have high Ni—P plating surface smoothness.
Further, in accordance with an increase of the magnetic disc in density, the magnetic region per bit is further miniaturized, and therefore even fine pits (pores) present in the plated surface of the magnetic disc cause an error upon reading data. For this reason, the plated surface of the magnetic disc is needed to be free of pits and have a high smoothness.
Under the circumstances, recently, an aluminum alloy substrate for magnetic discs having excellent plating surface smoothness has been strongly desired, and studies are being made for realizing such a substrate. For example, Patent Document 1 has proposed an aluminum alloy substrate for magnetic discs, in which the range of the aluminum alloy composition is limited to control the size of an Al—Fe or Mg—Si intermetallic compound, which is a cause of deterioration of the smoothness, to be optimum. Further, Patent Document 2 has proposed a method for improving the Ni—P plating surface smoothness, in which the annealing conditions are defined to control the number of Al—Mg—Zn intermetallic compounds.